Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. J-G. Zhu et al. has described another spintronic device called a spin transfer oscillator (STO) in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. The STO comprises a stack including a spin injection layer (SIL) with PMA character, an oscillating field generation layer (FGL) with in-plane anisotropy, and a spacer between the SIL and FGL.
Both MRAM and STT-MRAM may have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers typically referred to as a reference layer and free layer are separated by a thin non-magnetic dielectric layer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode in a MRAM device. In another aspect, a MTJ element in a read head sensor may be based on a giant magnetoresistance (GMR) effect that relates to a spin valve structure where a reference layer and free layer are separated by a metal spacer. In sensor structures, the MTJ is formed between two shields and there is a hard bias layer adjacent to the MTJ element to provide longitudinal biasing for stabilizing the free layer magnetization.
A high performance MRAM MTJ element is characterized by a high tunneling magnetoresistive (TMR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the change in resistance observed by changing the magnetic state of the free layer. A high TMR ratio and resistance uniformity (Rp_cov), and a low switching field (Hc) and low magnetostriction (λS) value are desirable for conventional MRAM applications. For Spin-MRAM (STT-MRAM), a high λS and high Hc leads to high anisotropy for greater thermal stability.
When a memory element uses a free layer with a magnetic moment lying in the plane of the film, the current needed to change the magnetic orientation of a magnetic region is proportional to the net polarization of the current, the volume, magnetization, Gilbert damping constant, and anisotropy field of the magnetic region to be affected. The critical current (iC) required to perform such a change in magnetization is given in equation (1):
                              i          c                =                                            α              ⁢                                                          ⁢              eVMs                                      g              ⁢                                                          ⁢              ℏ                                [                                    H                              k                                  eff                  ,                  ||                                                      +                                          1                2                            ⁢                              H                                  k                                      eff                    ,                    ⊥                                                                                ]                                    (        1        )            where e is the electron charge, α is a Gilbert damping constant, Ms is the saturation magnetization of the free layer, h is the reduced Plank's constant, g is the gyromagnetic ratio, Hkeff,∥ is the in-plane anisotropy field, and Hkeff,⊥ is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer. For most applications, the spin polarized current must be as small as possible.
The value Δ=kV/kBT is a measure of the thermal stability of the magnetic element. If the magnetization lies in-plane, the value can be expressed as shown in equation (2):
                    Δ        =                                            M              S                        ⁢                          VH                                                k                  eff                                ,                ||                                                          2            ⁢                          k              B                        ⁢            T                                              (        2        )            where kB is the Boltzmann constant and T is the temperature.
Unfortunately, to attain thermal stability of the magnetic region, a large net magnetization is required which in most cases would increase the spin polarized current necessary to change the orientation of the magnetic region.
When the free layer has a magnetization direction perpendicular to the plane of the film, the critical current needed to switch the magnetic element is directly proportional to the perpendicular anisotropy field as indicated in equation (3):
                              i          c                =                              α            ⁢                                                  ⁢                          eMsVH                                                k                  eff                                ,                ⊥                                                          g            ⁢                                                  ⁢            ℏ                                              (        3        )            
The parameters in equation (3) were previously explained with regard to equation (1).
Thermal stability is a function of the perpendicular anisotropy field as shown in equation (4):
                    Δ        =                                            M              S                        ⁢                          VH                                                k                  eff                                ,                ⊥                                                          2            ⁢                          k              B                        ⁢            T                                              (        4        )            
In both in-plane and out-of-plane configurations, the perpendicular anisotropy field of the magnetic element is expressed in equation (5) as:
                              H                                    k              eff                        ,            ⊥                          =                                            -              4                        ⁢            π            ⁢                                                  ⁢                          M              s                                +                                    2              ⁢                              K                U                                  ⊥                                      ,                    s                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              χ              ,              ⊥                                                          (        5        )            where Ms is the saturation magnetization, d is the thickness of the magnetic element, Hk,χ,⊥ is the crystalline anisotropy field in the perpendicular direction, and KU⊥,s is the surface perpendicular anisotropy of the top and bottom surfaces of the magnetic element. In the absence of strong crystalline anisotropy, the perpendicular anisotropy field of a magnetic layer is dominated by the shape anisotropy field (−4πMs) on which little control is available. At a given thickness, lower magnetization saturation decreases shape anisotropy and the spin-polarized switching current but also decreases thermal stability which is not desirable. Therefore, an improved configuration for a magnetic element is needed that provides improved thermal stability for a free layer with perpendicular magnetic anisotropy.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement and have a low switching current density but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is one of the key challenges for future MRAM applications and other spintronic devices. Theoretical expressions predict that perpendicular magnetic devices have the potential to achieve a switching current lower than that of in-plane magnetic devices with the same magnetic anisotropy field according to S. Magnin et al. in Nat. Mater. 5, 210 (2006).
When the size of a memory cell is reduced, much larger magnetic anisotropy is required because the thermal stability factor is proportional to the volume of the memory cell. Generally, PMA materials have magnetic anisotropy larger than that of conventional in-plane soft magnetic materials such as NiFe or CoFeB. Thus, magnetic devices with PMA are advantageous for achieving a low switching current and high thermal stability. Even as magnetic tunnel junctions (MTJs) lower the switching current by minimizing the demagnetization term, they provide a high energy barrier (Eb) due to the large perpendicular anisotropy maintained at small (<100 nm) junctions. Out of plane magnetic anisotropy begins to degenerate at annealing temperatures greater than about 350° C. and current thin films completely lose PMA character after 400° C. annealing processes. Thus, there is a significant challenge to increase PMA and enhance thermal stability of free layers to improve the performance of MTJs at elevated temperatures typical of back end of line (BEOL) semiconductor processes. Current technology fails to provide high Hc and thermal stability in a free layer with PMA character that will withstand high temperature processing up to at least 400° C. which is required in semiconductor fabrication methods. Therefore, an improved MTJ stack with a high thermally stable free layer that exhibits PMA is needed for magnetic device applications.